A diagram of the whole accelerator system is shown in FIG. 2. This apparatus is constructed of an entrance device 3 which enters charged particles, and a ring-shaped accelerator 50 which accelerates and stores the particles. Used as the injector 3 is a linac, a synchrotron, a microtron or the like. The ring-shaped accelerator 50 includes a beam duct 7 which forms a vacuum vessel for confining a particle beam 2, bending magnets 5 which deflect the orbit 10 of the particle beam 2, quadrupole magnets 6 which endow the particle beam with a focusing function, and a rf (radio frequency) accelerating cavity 4 which accelerates the particles.
For industrializing such an apparatus, it has become an important theme to reduce the size of the apparatus and yet to permit the storage of a large current. As one idea therefor, there is a proposal in which particles are entered at a low energy level below 100 MeV and are accelerated and then stored. Although there is an actual example having realized the proposal, a large current of about 500 mA has not been stored in any example yet. By the way, an apparatus of this type is discussed in, for example, "Institute of Physics, Conference Series No. 82, p. 80-84 (Cambridge, 8-11 Sept. 1986)".
In the ring-shaped accelerator, the particles circulate while betatron-oscillating round a closed orbit corresponding to the energy of the particles. Besides, as shown in FIG. 3, the bunch of particles to be accelerated have as their central orbit a closed orbit 20 which corresponds to their center energy. In general, a closed orbit 21 corresponding to energy higher than the center energy lies outside the central orbit 20, whereas a closed orbit 22 corresponding to energy lower than the center energy lies inside the central orbit 20. In this manner, the closed orbits of the particles exhibit energy dispersiveness.
On the other hand, in order to accelerate the bunch of particles, at least one rf accelerating cavity is disposed on the orbit of the particles, so that the particles are oscillated also in terms of energy by the acceleration/deceleration mechanism of a rf electric field based on the cavity. This phenomenon is usually called "synchrotron oscillations". The synchrotron oscillations affect the betatron oscillations of the particles on account of the energy dispersiveness of the closed orbit stated above. For this reason, the amplitude of the transverse oscillations of the particles enlarges with the spread of an energy distribution attributed to the synchrotron oscillations.
Thus, the beam widens greatly in the transverse direction thereof The widening gives rise to a transverse wake field (a transient electromagnetic field due to the interaction between the particles and the wall of the vacuum vessel), and the wake field renders the behavior of the particle bunch unstable. Heretofore, this phenomenon has led to the problem that a heavy beam loss arises in the acceleration process of the particles after the injection thereof, so the storage of the large current is impossible.